Our NIR spectroscopic observations have revealed that in addition to dust-formation, dLHdC and RCB stars can in most cases be distinguished based on their values of \(^{16}\) O/\(^{18}\) O. dLHdC stars in general have a lower \(^{16}\) O/\(^{18}\) O than RCB stars. It is not surprising that the oxygen isotope ratios are an important factor in the study of dLHdC and RCB stars. Anomalously low \(^{16}\) O/\(^{18}\) O in dLHdC and RCB stars was key to identifying the merger of a He-core and a CO-core white dwarf as their formation channel. \(^{18}\) O is synthesized by the partial helium burning reaction \(^{14}\) N(\(\alpha \) ,\(\gamma \) ) F\(^{18}\) (\(\beta ^{+} \nu \) )\(^{18}\) O. This reaction is efficient at temperatures of \(\approx 10^{8}\)  K [1]}, [2]}. At higher temperatures, \(^{18}\) O is burnt to \(^{22}\) Ne. These conditions can be achieved in a thin helium burning shell around the merger remnant of a He-core and a CO-core white dwarf [1]}. The \(^{18}\) O is convectively dredged up to the surface of the star within the first few hundred years after merger [4]}, [5]}, [6]}. The photospheric value of \(^{16}\) O/\(^{18}\) O in dLHdC and RCB stars is thus set within the first hundred years and remains constant for the rest of their lifetimes (\(\approx 10^{4-5}\)  years). Here, we explore the properties of the merging white dwarfs that set the values of \(^{16}\) O/\(^{18}\) O in the remnant supergiants. We also discuss the implications of our observations on the dLHdC-RCB connection.
